1. Field of the Invention
The present invention relates generally to the treatment of wastewater and more particularly to increasing the level of oxygen in oxygen depleted wastewater to facilitate the aerobic treatment of wastewater in a lagoon.
2. Description of Related Art
It is well known that certain kinds of waste can be treated by relatively long-term exposure of bacteria in lagoons. Typically, the treatment of waste in lagoons is both aerobic and anaerobic, that is the treatment is by bacteria that require oxygen as well as bacteria that do not require oxygen to break down organic material in the lagoons. The byproducts of anaerobic bacteria include hydrogen sulfide and the like, which have an unpleasant odor and make the lagoons bad neighbors. Aerobic bacteria produce carbon dioxide as a byproduct and therefore can be situated in populated areas without creating noxious odors.
In many instances, waste created by food processing plants and the like has such a low oxygen content that only anaerobic bacteria activity can be supported in the waste. It has become common to attempt elevating the level of oxygen mixed in the waste by a variety of methods. In one method an aerator is floated on the surface of the lagoon. This floating or xe2x80x9csurfacexe2x80x9d aerator is in effect an agitator that causes a violent splashing of the water wherein water at or just below the surface of the lagoon is ejected into the air. All of the splashing and ejection of the water increases the exposure of the water to air and aids in increasing the oxygen content of the water which enhances the ability of the water to support aerobic activity.
A drawback of surface aerators is that they are relatively inefficient in terms of the amount of oxygen introduced into the wastewater per unit of energy consumed. In this respect a large amount of energy is required to drive the impellers of a surface aerator through the water and the introduction of oxygen is limited to the upper reaches of the wastewater lagoon. Accordingly, the cost for obtaining even a modest increase in oxygen content in the immediate area of the surface aerator is relatively high. Also, the enhanced oxygen effects are transient in that the oxygen concentration in the wastewater returns to the non-aerated level almost immediately when surface aeration stops. The aerobic activity quickly ceases as anaerobic activity with its associated problems resumes.
In another apparatus and method, air is introduced towards the bottom of the lagoon from a bubble pipe. The pipe generally is a horizontal pipe having a plurality of drilled holes. Air pumped into the pipe escapes through the holes and rises in streams of bubbles to the surface. The rising streams of bubbles act to increase the oxygen content in the immediate area. For enhanced efficiency, the bubble pipe can be supported for movement across the bottom of the lagoon to increase the volume of wastewater exposed to the bubbles. While bubble aeration is effective to some extent, it suffers the same defect as surface aeration in that the effects of enhanced oxygenation are transient. The relatively large bubbles quickly rise to the surface and an elevated oxygen level quickly falls as soon as the pumping of air to the bubble pipe stops.
The amount of oxygen in wastewater is conventionally measured in milligrams per liter. The wastewater produced by food processing plants may have no more than a few 10ths of a milligrams/liter of oxygen at the time it is introduced into a treatment lagoon. The aerators just described may increase the oxygen concentration in the water to 5 or 6 milligrams/liter in the areas directly affected by the aerators but the increase is very localized and is maintained only as long as the aerators are actually running.
Aerators of the type described are inefficient in terms of the amount of oxygen introduced into the wastewater per unit of energy consumed. For example, it is not unusual for even a moderate sized treatment facility to use $100,000 or more of electricity per year to power the surface aerators necessary to produce even a modest increase in aerobic bacterial activity. In both types of aeration systems, the event of a power failure will cause the oxygen concentration of the water to return to the non-aerated level almost immediately and aerobic activity quickly ceases as anaerobic activity with its associated problems resumes.
In a more recent innovation, a centrifuge has been used to introduce larger amounts of air into the wastewater. In this case the untreated wastewater is pumped to a centrifuge. Water introduced into the center of the centrifuge is hurled outward by centrifugal force to impinge on a wall of the centrifuge. This breaks the water stream into small droplets and increases the surface area to enhance the uptake of air into the wastewater stream by decreasing bubble diameter. By significantly reducing the size of the bubbles formed in the wastewater to a micro-fine size (less than 15 microns), the amount of oxygen absorbed into the water at the bubble interfaces increases. The residence time of the bubbles in the wastewater also increases, as the smaller bubbles are slower to rise to the surface.
The oxygen-enriched stream then is the fed back into the lagoon where the oxygen enhanced stream mixes with the untreated wastewater. While this method has advantages over oxygen enriching systems of the prior art, a still further improved and more efficient system is desired.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for elevating the oxygen content of wastewater.
Another object is to provide a method and apparatus for aerating wastewater that allows the wastewater to retain an elevated oxygen content for a substantial time after aeration stops.
A further object is to provide a method and apparatus for aerating wastewater by creating a mist composed of water droplets thoroughly mixed with atmospheric air, coalescing the droplets to entrap the air in a treated stream and introducing the treated aerated stream into a wastewater lagoon.
In the present invention, an aeration method and apparatus is provided that increases the oxygen content of wastewater to levels that remain relatively high over time after aeration stops. This renders the oxygen content less transient so that short interruptions in the operation of the apparatus can be tolerated without adversely effecting the ability to support aerobic treatment of the wastewater.
Briefly stated and in accordance with the method of the present invention, a stream of the untreated wastewater is drawn from a treatment lagoon. The stream is aerated to increase its oxygen content and then is discharged back into the lagoon. Aeration is conducted in a manner such that the oxygen content of the water remains elevated for a substantial time after discharge.
The oxygen content of the wastewater is increased by introducing a stream of the wastewater into a vessel containing a rapidly spinning rotor comprising a plurality of vanes. Preferably the vessel is land based although it may be placed on a float in the lagoon. The stream entering the vessel is directed in a path parallel to the axis of rotation of the rotor and into the path of the rotor vanes. The rotor is driven at speeds sufficient to cause the vanes to impinge against the stream with enough force to cause the stream to disintegrate into a mist of water droplets. A slight negative pressure is created in the vessel as the surface area of the stream expands so atmospheric air is drawn into the vessel and entrained in the water droplets. As the droplets coalesce and reform after impacting the vanes, micro-fine bubbles of air are formed and entrapped. In this fashion the wastewater stream in the vessel is aerated.
A stream of the wastewater, now aerated, is discharged from the vessel by the pumping action of the rotor and is directed back into the lagoon. In the lagoon, the aerated stream dissipates to enhance the oxygen content of the wastewater in the lagoon.
It has been noted that the oxygen content of wastewater treated in this manner remains elevated for a significant time after the treatment stops. This is attributed in part to the micro-fine size of the oxygen carrying bubbles created by impinging the vanes of the rotor against the wastewater stream. These small bubbles not only increase the volume of air in the water but also remain in the water for an extended period so the wastewater can absorb a significant amount of oxygen.
It is preferred that the vanes be perforated. These perforations expose the impinging wastewater stream to a greater number of edge surfaces, which is believed to facilitate shearing the stream and dramatically increasing the amount of oxygen introduced into the wastewater.
Accordingly, the present invention may be characterized in one aspect thereof by a method of aerating wastewater to enhance aerobic treatment comprising:
a) introducing a stream of oxygen depleted wastewater from a lagoon into an upper level of a treatment vessel and directing the stream downwardly in the vessel and into the path of perforated vanes rotating about a vertical axis in the vessel;
b) impinging the vanes at a right angle against the downwardly flowing wastewater stream with sufficient force to temporarily divide at least part of the wastewater stream into a fine droplet mist, the dividing of the wastewater stream into the fine droplet mist and resulting surface area expansion creating a negative pressure within the vessel;
c) drawing atmospheric air in to the vessel in response to the negative pressure, the air drawn into the vessel intimately mixing with the fine droplet mist and becoming entrained in the mist and forming relatively small bubbles as droplets in the mist coalesce thereby facilitating the dissolving of atmospheric oxygen into the wastewater and enhancing the oxygen content of the wastewater;
d) expelling the now oxygen enhanced treated wastewater from an outlet at a lower level of the vessel under the influence of a pumping action produced by the rotating vanes; and
e) returning the oxygen enhanced wastewater back to the lagoon.
In another aspect, the present invention may be characterized by an apparatus for aerating wastewater comprising:
a) a vessel having an oxygen depleted wastewater inlet at an upper level and an oxygen enhanced wastewater outlet at a lower level;
b) a vaned rotor within the vessel and arranged for rotation about a vertical axis, each of the vanes having a plurality of openings therethrough;
c) means for rotating the rotor;
d) the inlet being arranged to direct an oxygen depleted wastewater stream downwardly into the vessel along a path generally parallel to the rotor axis of rotation so that the vanes impinge against the oxygen depleted wastewater stream at a generally right angle thereby dividing the stream into a fine droplet mist and creating a negative pressure in the vessel;
e) a valved inlet for admitting atmospheric air into the vessel in response to the negative internal pressure, the air mixing with the fine droplet mist thereby enhancing the oxygen content of the wastewater stream; and
f) the rotating vaned rotor providing a pumping means to expel the now oxygen enhanced wastewater stream from the enhanced wastewater outlet at a lower level of the vessel.